The invention relates to an acetabular unit for prostheses, mainly for hip prostheses, which is provided with a conical basket made of a tissue-compatible metal alloy and can be anchored in the cotyloid cavity without bone cement. The invention relates also to a device for fixing said unit in the cotyloid cavity.
About four thousand prostheses, most of them hip prostheses, are implanted daily in the world. The number of implantations is ever-increasing because, as a consequence of speedy increase of average human age the number of diseases due to the wear of the joints increase proportionally. Dynamic development of surgical treatment and surrounding fields has further increased the number of implantations. At the same time, several deficiencies of the routine proceedings can be definitely outlined.
Joints are composed of two components: the spherical joint head of the neck part of the bone which is matched to the concavely shaped "counterpart" of the bone, the cotyloid cavity. Both surfaces destroyed by different pathological conditions necessitate that they be replaced. So e.g. in case of a hip-joint, the surgeon first exposes the body region and removes the head, and neckpart of the femur up to the plane between the buttocks, thereafter, he replaces the removed part with a biomechanically expediently formed prosthesis made of a tissue-compatible metal and having been chosen from a variety of different sizes and shapes of the femurs medullary cavity thus exposed. The prosthesis is fixed by a bone-cement or mechanically, thereafter, the counterpart made of a tissue-compatible synthetic material is fixed in the hollowed cavity of the hip-bone. By matching the artificial acetabular unit we obtain the ball-and-socket joint: the hip-prothesis.
Out of the most complicated partial problems accompanying the operation described above in a simplified way, one of the most frequently occuring complications to be concerned with is the loosening of the hip-acetabulum followed by the release of the whole artificial joint. Viewing the phenomenon from the point of view of biomechanics: the reasons of inadequate fixation of the prothesis, i.e. the acetabular unit and the consequent loosening thereof are due to both biological and mechanical reasons.
Biologically the problem is mainly the properties of the cementing means. Methyl acrylate is applied for this purpose, being a catalyst when polymerized at room temperature. The polimerizate is however not a complete polymer and its free monomers are tissue-destructive. At the same time it does not react at "room temperature" and exothermal heat formation may reach even 75.degree. C., accordingly it denatures living proteins. Irrigation with a cold liquid during the course of surgical procedure is not the solution, as both the synthetic acetabulum and the acrylate are good heat-insulators. Resulting in bone destruction and the prosthetic loosening.
Mechanical problems can be explained by classical anatomic conditions: whereas the conically widening end of the tubular femur enables a relatively stable fixatin of the prosthesis, the hip-cavity is shallow and concave, as a consequence, the hemi-spherical prosthesis, which is glued-in is not as closely congruent to the anatomical site allowing it to get displaced easily under the combined effect of the shearing and compressive forces corresponding to the body weight.
Essentially the same situation must be confronted in the case of the other acetabular prostheses. It has been tried to eliminate biological deficiencies by simply omitting the bone-cement and fixing the prostheses mechanically. As previously stated, this may be acceptable for the joint-heads but is accompanied with difficulties in the cotyloid cavities.
Accordingly, experiments are carried out to eliminate said difficulties. One of the suggestions is to coat the outer surface of the metal acetabular basket embedding the acetabulum with a tissue-compatible, so-called bioactive material, which is building into the bone material and ensures mechanical anchoring.
According to another method the acetabular basket is provided with an irregular porous "bioactive" surface, into which the embedding bone surface is built-in after a certain time.
Although these solutions yield a far safer bond, when compared to earlier solutions, their fundamental drawback is that the patient post surgery is confined to bed during the long-lasting process of incorporation which may last for several weeks. Considering, that the prostheses used are usually for an elderly patient population, long confinement to bed after the surgical intervention increases considerably risks of operation pneumonia, thrombosis, decubitus ulcer etc.)
It is also known the form on the outer surface of the acetabular basket is a screw-thread or thread segments and the basket is screwed into the prepared acetabulum bed. Prior to anchoring a thread is cut in the acetabular cavity or a self-cutting formation is used.
Generally, a threaded acetabular basket is of a conical shape. Accordingly, it is unable to penetrate into the pelvis cavity. At the same time, due to its cone angle, the proper wedged state can be achieved, and compared to earlier solutions it is more resistant to tilting. However, the danger remains that it is easily torn out from the disintegrated spongy bone substance.